Car Won't Accelerate: Common Causes & Codes

Why Your Car Won't Accelerate

Few things are more frustrating and dangerous than pressing the gas pedal and getting little or no response. When your car won't accelerate properly, it usually means the engine is not getting the right amount of air, fuel, or spark, or the transmission is not transferring power to the wheels correctly. Modern vehicles use electronic throttle control, meaning there is no physical cable between the gas pedal and the throttle body. Instead, sensors tell the computer how far you are pressing the pedal, and the computer opens the throttle accordingly. If any sensor in this chain fails, the computer may put the engine into a reduced power mode, sometimes called limp mode, which limits your speed to around 25 to 35 mph to prevent damage.

Throttle and Sensor Problems

The throttle position sensor (TPS) tells the engine computer how far open the throttle is. Codes P0120, P0121, P0122, and P0123 all relate to TPS problems. P0120 means the throttle position sensor circuit has a malfunction. P0121 means the sensor reading is out of expected range. P0122 indicates the signal is too low, and P0123 means it is too high. When the computer cannot trust the throttle position reading, it will often enter limp mode and severely limit acceleration. A dirty or failing throttle body can cause similar symptoms. Carbon buildup on the throttle plate can prevent it from opening fully. Cleaning the throttle body with throttle body cleaner is a $10 DIY fix that solves a surprising number of acceleration problems. Codes P2106 and P2110 indicate the throttle actuator control system has been forced to a limited position, which is the computer confirming it has put the engine into reduced power mode.

Fuel and Air Delivery Issues

Your engine needs the correct ratio of fuel to air for proper combustion. Code P0171 means bank 1 is running lean, meaning too much air or not enough fuel. P0172 means bank 1 is running rich, meaning too much fuel. Both conditions can cause poor acceleration. A lean condition is often caused by a vacuum leak, a dirty mass airflow (MAF) sensor, or a weak fuel pump. Cleaning the MAF sensor with MAF-specific cleaner is another easy $10 fix. A failing fuel pump may deliver enough fuel at idle but cannot keep up with demand during acceleration. You might notice the car hesitates or stutters when you press the gas hard. A clogged catalytic converter can also cause poor acceleration because exhaust gases cannot exit the engine fast enough, creating back-pressure that chokes the engine. You might notice the car feels like it hits a wall at a certain RPM.

Misfires and Transmission Problems

Code P0300 indicates random or multiple cylinder misfires, which directly reduces the power your engine can produce. If three of your four cylinders are firing but one is not, you are losing 25 percent of your engine's power. Worn spark plugs, bad ignition coils, or clogged fuel injectors are the typical causes. Transmission problems can also feel like the engine won't accelerate. If the transmission is slipping, the engine revs up but the car does not speed up accordingly. You might notice the RPM gauge climbing without a corresponding increase in speed. Low transmission fluid, worn clutch packs, or a failing torque converter can all cause this. Check your transmission fluid level and condition. If it is dark brown or smells burnt, the transmission needs service.

What to Do When Your Car Won't Accelerate

If your car suddenly loses power while driving, turn on your hazard lights and carefully move to the right lane or shoulder. If the check engine light is on, get the codes scanned as soon as possible since the codes will tell you exactly which system is failing. Many auto parts stores scan codes for free. If your dashboard shows a reduced power or limp mode warning, you can usually still drive at low speeds to get to a safe location, but do not try to force the car to go faster. Start your diagnosis with the cheapest and easiest checks. Clean the MAF sensor and throttle body, check the air filter, and inspect spark plugs if they have not been changed in 60,000 miles or more. Check for vacuum leaks by listening for hissing sounds around the intake manifold with the engine running. If the problem is intermittent, it may be a sensor that is failing under certain conditions. An OBD-II scanner that shows live data can help you watch sensor readings in real time to catch the problem as it happens.

Frequently Asked Questions